The present invention generally relates to corona generating devices and, more particularly, to a charging device having a DC portion followed by an AC portion for use in electrophotographic printing machines.
Electrophotographic marking is a well known and commonly used method of copying or printing documents. In general, electrophotographic marking employs a charge-retentive, photosensitive surface, known as a photoreceptor, that is initially charged uniformly. In an exposure step, a light image representation of a desired output focused on the photoreceptor discharges specific areas of the surface to create a latent image. In a development step, toner particles are applied to the latent image, forming a toner or developed image. This developed image on the photoreceptor is then transferred to a print sheet on which the desired print or copy is fixed.
The electrophotographic marking process outlined above can be used to produce black and white (monochrome) as well as color images. In general, color images are produced by repeating the electrophotographic marking process once for each color of toner used. Several methods exist for repeating the electrophotographic marking process to obtain color images. In one such method, the uniformly charged photoreceptor is initially exposed to a light image which represents a first color, such as black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. This same image area with its black toner layer is then recharged, exposed, and developed to produce a second color toner layer, such as yellow. This recharge/expose/and develop image on image (REaD IoI) development process may be repeated to subsequently develop images of different colors, such as magenta and cyan. The images may be formed by using a single exposure device, e.g. a raster output scanner (ROS), where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
In generating color images using the REaD IoI process the photoreceptor must be recharged to a substantially uniform level prior to the exposure and development of the next toner layer. When recharging the photoreceptor, it is important to level the voltages among the previously toned and untoned areas of the photoreceptor. While it may be possible to achieve voltage uniformity by simply recharging previously toned areas to the same voltage level as untoned areas, residual toner voltage complicates the process.
Residual toner voltage is the residual charge and the resultant voltage drop that exists across the toner layer of a developed (toned) area of the photoreceptor. The residual charge associated with previously developed toner images reduces the effective development field in the toned areas, affecting the consistency and desired uniformity of the developed mass of subsequent toner images. Color quality is severely threatened by the presence of the residual charge and the resultant voltage drop across the toner layer. The change in voltage due to the toned image can be responsible for color shifts, increased moire, increased color shift and toner spreading at image edges. The problem becomes increasingly severe with each additional toner layer due to the increased toner mass which must be neutralized prior to the exposure and development of the next toner layer.
Furthermore, recharging the photoreceptor becomes more difficult as the speed of the printing machine increases. To enable printing machines to increase the number of prints per minute, the speed at which the photoreceptor travels is typically increased. This increase in photoreceptor speed reduces the amount of time that a charging device acts upon the photoreceptor to recharge the photoreceptor and minimize the residual voltage associated with the toned areas.
The following references may be found relevant to the present disclosure.
U.S. Pat. No. 5,258,820 to Tabb discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development of a charged area (CAD), and a pre-recharge corona device is used following development of a discharged area (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
U.S. Pat. No. 5,539,501 to Yu et al. discloses a corona generating device which includes a shell, a plurality of corona wires within the shell and a power source that outputs first and second alternating voltages which are out-of-phase with each other. The corona wires are connected to the power source such that the voltage received at each wire is out-of-phase with the voltage at adjacent wires.
U.S. Pat. No. 5,579,100 to Yu et al. discloses a recharging step employed between two image creation steps. The recharging step utilizes a corona generating device to recharge developed and untoned areas of a charge retentive surface to a lower potential than that associated with the developed areas before the recharge step.
U.S. Pat. No. 5,581,330 to Pietrowski et al. discloses a multi-color printing machine utilizing a recharge step between two image creation steps for conditioning a charge retentive surface pursuant to forming the second of the two images. The printing machine includes a voltage sensitive corona generating device having a high characteristic slope described in a graph of the current delivered to a charge receiving surface (I) vs. grid minus charge receiving surface voltage (V) to both reduce the residual toner voltage across the previously toned image, and to charge the toned and untoned areas of the charge retentive surface to a substantially uniform level.
U.S. Pat. No. 5,600,430 to Folkins et al. discloses a multi-color imaging apparatus utilizing a split recharge configuration wherein a first corona generating device recharges a charge retentive surface having a developed image thereon to a higher absolute potential than a predetermined potential, and then an alternating current second corona generating device recharges the surface to the predetermined potential.